This invention concerns a method of making a rigid polyurethane foam that is cost efficient and which yields a product that is relatively light in weight, yet has good crush resistance.
Polyurethane resins are made by reacting polyols with polyisocyanates. The reaction is exothermic. Cross-linking, or branching, of the polyurethane molecules can be achieved by including in the reaction mixture some polyol molecules and/or isocyanate molecules that have at least three functional groups, and by adjusting the ratio of reactants accordingly. With sufficient cross-linking, rigid, thermoset polymers are obtained.
To make rigid polyurethane foam, a mixture is made of a polyfunctional isocyanate, a polyol, a blowing agent, a catalyst, and, usually, a cell-size regulator (e.g., a surfactant). A urethane-forming reaction begins once the ingredients are combined, an exotherm forms, and the blowing agent or agents cause closed cells to form in the polymer as the mass expands and solidifies. The exotherm typically reaches a peak temperature of at least about 150xc2x0 F. The isocyanate and polyol reactants include enough molecules with three or more functional groups that the degree of cross-linking or branching is sufficient to produce a rigid foam.
Aromatic polyisocyanates often are used when making rigid foam. Some examples are toluene diisocyanate (TDI) and polymeric isocyanate (PMDI), which is obtained by the condensation of aniline with formaldehyde.
Polyols that can be used include polyether polyols and polyester polyols. Propylene oxide adducts of polyfunctional hydroxy compounds or amines are one type of polyether polyols that can be used. Mixtures of polyester polyols and polyether polyols sometimes are employed.
Halogenated hydrocarbons, such as hydrochlorofluorocarbons and hydrofluorocarbons, can be used as blowing agents. Lower alkanes such as pentanes and cyclopentanes can be used as well. Water can also be used, as it will react with isocyanate to generate carbon dioxide in situ. Sometimes water or carbodimide catalysts are used to generate carbon dioxide as a co-blowing agent. Often the blowing agent or agents are preblended with the polyol, together with the catalyst and the cell-size regulator, which usually is a surfactant.
All of this is well known to persons of ordinary skill in the art and is described, for example, in Kirk-Othmer Encyclopedia of Chemical Technology, 4th Ed. (1997), vol. 24, pp. 695-715, which is incorporated herein by reference.
The cost of the reactants to make rigid polyurethane foam is relatively expensive. It has now been discovered that the cost per volume unit of the finished foam can be lowered by including in the polyurethane reaction mixture expandable polystyrene beads (EPS) that are made of a polystyrene that has a softening point that is equal to or below the maximum temperature reached by the exotherm during the urethane-forming reaction. The heat of the urethane-forming reaction causes the polystyrene beads to expand and sinter, while trapped in the foaming matrix of polyurethane. Since the expanded polystyrene beads are less expensive, on a volume basis, than the foamed polyurethane, the materials cost of the polystyrene-containing foam is less than that of the same volume of straight polyurethane foam. The monetary savings can be substantial, as the cost ratio of polyurethane foam to expanded polystyrene, on a volumetric basis, is currently about 25/1 to 30/1.
A side benefit of including the EPS beads as a filler in the polyurethane is that the resultant foam is less dense than unfilled polyurethane. Yet another benefit is that the polystyrene-containing foam, despite the presence of the relatively low-cost filler (the expanded polystyrene), appears to have the same or better crush resistance as the unfilled polyurethane.
Expandable polystyrene beads often are used to form lightweight molded objects. Typically they are pre-foamed by heating to a temperature above their softening point, which often will be in the range of about 165-185xc2x0 F., until they foam to give a loose aggregate of the desired bulk density. The pre-foamed particles may then be placed in a mold or other cavity and heated with live steam, causing them to sinter and fuse together to form a lightweight solid whose dimensions correspond to those of the mold cavity.
When present in the reaction mixture that forms a rigid polyurethane foam, the EPS beads, as mentioned above, expand and sinter. This creates gas-filled pockets, of various sizes, in the foam, which are lined with polystyrene. It appears that isolated spherical beads generate relatively spherical pockets. In some sense, it might be said that there are thin-walled polystyrene globules dispersed throughout the polyurethane foam, which are up to two or three times larger than the EPS beads from which they are formed.
Moreover, it appears that these polystyrene globules are coated with a layer of xe2x80x9cskinnedxe2x80x9d polyurethane, i.e., a thin continuous layer of the polyurethane, much as is present on the outer surface of rigid polyurethane foam moldings. The presence of these double-walled hollow structures seems to enhance the foam""s crush resistance.
Methods of making expandable polystyrene beads are well known. As disclosed in U.S. Pat. Nos. 3,991,020; 4,287,258; 4,369,227; 5,110,835; 5,115,066; and 5,985,943, for example, all of which are incorporated herein by reference, EPS beads may be made by polymerizing styrene in an aqueous suspension, in the presence of one or more expanding agents that are fed at the beginning, during, or at the end of polymerization. Alternatively, they may be made by adding an expanding agent to an aqueous suspension of finely subdivided particles of polystyrene.
The expanding agent, also called a xe2x80x9cblowing agent,xe2x80x9d is a gas or liquid that does not dissolve the styrene polymer and which boils below the softening point of the polymer. Examples of suitable blowing agents include lower alkanes and halogenated lower alkanes, e.g., propane, butane, pentane, cyclopentane, hexane, cyclohexane, dichlorodifluoromethane, and trifluorochloromethane. Often the beads contain about 3 to 15%, based on the weight of the polymer, of the blowing agent. Preferably, the blowing agent will be present at a level of about 3 to 7%.
By xe2x80x9cbeadsxe2x80x9d we here mean small particles of any geometry, e.g., spherical, cylindrical, or lumpy. By xe2x80x9cpolystyrenexe2x80x9d is here meant a styrene homopolymer or copolymer containing 50 wt. % or more, preferably at least 80 wt. %, of styrene. Examples of suitable comonomers are xcex1-methylstyrene, ring-halogenated styrenes, ring-alkylated styrenes, acrylonitrile, esters of acrylic or methacrylic acid with alcohols having from 1 to 8 carbon atoms, N-vinylcarbazole, and maleic acid or anhydride. A minor amount of a copolymerized chain-branching agent may be included in the polymer as well. Suitable such agents are compounds containing at least two xcex1, xcex2-ethylenically unsaturated groups, such as divinyl benzene, butadiene, and butanediol diacrylate. Branching agents are generally used in an amount of about 0.005 to 0.05 mol %, based on the styrene.
The polystyrene in the EPS beads usually has a weight average molecular weight in the range of about 130,000 to about 300,000.
Expandable polystyrene beads may contain other additives to impart specific properties either to the beads or to the expanded products. These include, for example, flameproofing agents, decomposable organic dyes, lubricants, fillers, and anti-agglomerating additives. Depending on the intended effect, the additives may be homogeneously dispersed in the beads or present as a surface coating.
EPS beads come in different unexpanded particle sizes. Generally, the beads"" longest dimensions will be in the range of about 0.1 to 6 mm, often about 0.4 to 3 mm. It is thought that unexpanded particle sizes in the range of about 0.4 to 1.6 mm are preferred for the present invention.
EPS beads vary as to their expansion capability, i.e., how large they can get when heated to expansion temperature. The expansion capability of an EPS bead can be reported in terms of the bulk density of the loose aggregate the beads will form when they are fully expanded (xe2x80x9cfully expanded densityxe2x80x9d). By xe2x80x9cfully expandedxe2x80x9d is here meant the expansion that results from the xe2x80x9ctwo passxe2x80x9d expansion process described in Example 2 of U.S. Pat. No. 5,115,066. This entails the use of a Tri Manufacturing Model 502 expander (or equivalent), operated at an inlet steam temperature of about 211xc2x0 F. and an inlet steam flow rate of approximately 74 pounds per hour. The first-pass throughput rate is about 208 pounds per hour. A fluidized bed drier, blowing ambient air, is used to cool the resulting prepuff. After aging for 3 hours at ambient temperature and humidity, the prepuff is run through the expander again, under the same conditions, except operating at a throughput rate of about 217 pounds per hour.
It is thought that EPS beads having a fully expanded density in the range of about 0.5 to 4.5 pounds per cubic foot (pcf) are preferred in the present invention.
The polystyrene beads may be added to one of the polyurethane. reaction mixture components prior to mixing the components, or the beads may be added just after the components are mixed, e.g., while the exothermic reaction is taking place. It is believed that the apparatuses and processes disclosed in U.S. Pat. Nos. 5,152,943 and 5,547,276, each of which is incorporated herein by reference, are effective for adding expandable polystyrene beads to a component. It is believed that the process disclosed in U.S. Pat. No. 6,065,862, which is incorporated herein by reference, is effective for adding expandable polystyrene beads to the mixture. Of course, one of ordinary skill will appreciate that other apparatuses and processes may be used.
The amount of expandable polystyrene beads added to the polyurethane reaction mixture in the present process preferably is enough to lower the density of the resultant foam, but not so much as to prevent the polyurethane from being the continuous phase in the finished foam. By xe2x80x9clower the densityxe2x80x9d is meant that had the same polyol and isocyanate reactants been used, in the same amounts, but without any of the EPS beads, the density of the resultant polyurethane foam would have been greater than what resulted when the EPS beads were present. In one free rise experiment, for example, by adding 4 wt. % EPS beads, the density of the rigid polyurethane foam was lowered from 5.5 pcf to 4.95 pcf.
It is believed to be generally preferred that the amount of the expandable polystyrene beads be about 5% or less, e.g., about 1 to 4%, based on the combined weight of the polyurethane reactants. Amounts greater than 4 or 5 wt. % may be difficult to uniformly mix into the polyurethane reactants.
The reaction and foaming of the EPS-containing reaction mixture can take place in an unrestricted space or in an enclosure, such as a mold, that will confine the mixture of foaming and expanding ingredients. When the reaction and foaming occurs in a confined enclosure, it is contemplated that the volumetric ratio of void space to reactants in the enclosure will be such as to generate during the foaming process a maximum pressure of at least about 20 or 30 psi, and ranging up to about 70 psi.
The term xe2x80x9cpolyurethane systemxe2x80x9d can be used to refer to a particular combination of isocyanate, polyol, catalyst, blowing agent, and cell size regulator. A characteristic that helps identify and distinguish polyurethane systems is the density of the foam a particular system will create when the components are mixed in an open vessel (the xe2x80x9cfree rise densityxe2x80x9d). It is thought that polyurethane systems having a free rise density of about 3 or 4 pcf to 20 or 25 pcf are generally preferred for use in the present invention.
Examples of some commercial isocyanate/polyol pairings that can be employed in forming polyurethane systems for use in the present invention are the following:
In the above table, the Rubinate and Rimline reactants are available. from Huntsman Chemicals, and the Baydur reactants are available from Bayer Corporation.
It is thought to be generally preferred, when practicing the process of the present invention, that polyurethane reactants be used that will generate an exotherm having a peak temperature in the range of about 185-285xc2x0 F.